1. Field of the Invention
The present invention relates to a raw material for manufacturing a fluoride crystal, a method of refining the raw material, a fluoride crystal, a method of manufacturing the fluoride crystal and an optical part, and more specifically to a raw material suited for manufacturing a fluoride crystal suited for various kinds of optical elements, lenses, window materials, prisms, etc. to be used in a broad wavelength range from the vacuum ultraviolet region to the far infrared region, in particular a calcium fluoride crystal to be used as an optical part for an excimer laser, a method of refining the raw material, a fluoride crystal and an optical part.
2. Related Background Art
Fluoride crystals such as calcium fluoride, which has high transmittance within a broad range of wavelengths from the vacuum ultraviolet region to the far infrared region, are widely utilized as various kinds of optical elements, lenses, window materials, prisms and so on.
Furthermore, fluorite (which exhibits an excellent transmission characteristic at short wavelengths) is useful as an optical member for excimer lasers, and in particular as a lens with a diameter of 200 mm and more. Crystals of calcium fluoride (which exhibits internal transmittance of 70% or higher for a light with a wavelength of 135 nm) has excellent durability against an ArF excimer laser and its transmittance hardly degrades even when it is repeatedly irradiated with a laser with a high output.
Such a fluoride crystal tends to become turbid white or opaque when only the raw material for the crystal is melted and crystallized by moving a crucible in a furnace with a temperature gradient.
(1) In order to obtain a crystal with an excellent transmission characteristic, it is therefore necessary to prevent oxidation of the raw material of the crystal and add a scavenger to remove impurities.
A technique has been developed to use lead fluoride as a scavenger (Stockbarger, J. Opt. Soc. Am. 39, (1949) pp. 731-740) as well as a technique to use cadmium fluoride as a scavenger (Radzhabov and Figura, Phys. Stat. Sol. (b) 136 (1986) pp. k55-k59).
(2) Furthermore, an attempt has been made to refine and grow a crystal by melting a raw material in the atmosphere of a reactive gas instead of a scavenger and then gradually cooling it. Hydrogen fluoride gas diluted with helium is used as a reactive gas (Guggenheim, J. Appl. Phys. 34, pp. 2482-2485 (1963), Robinson and Cripe, J. Appl. Phys. 37, pp. 2072-2074 (1966), etc.), which is, a mixture gas consisting of hydrogen fluoride gas, tetrafluoromethane, sulfur tetrafluoride and boron trifluoride (Pastor, Robinson and Braunstain, Mat. Res. Bull. 15, pp. 469-475 (1980)) as well as a cracked product gas of teflon (Chernevskaya and Korneva, Opt. Tech. 39, pp. 213-215 (1972)).
When a scavenger is used, however, metal elements constituting the scavenger remain in the crystal. Since metal elements which remain in larger amounts have a high possibility of having an adverse influence on the transmission characteristic, it is necessary to reduce the amount of metal elements that remain in the crystal by adding a smaller amount of the scavenger. When the scavenger is added in an amount that is too small, however, it reduces a scavenging effect, thereby allowing the transmission characteristic to degrade drastically due to oxidation, etc. of the raw material for the crystal. It is therefore important to define the optimum addition amount of the scavenger. However, the optimum addition amount varies depending on the water concentration and the content of impurities in the raw material for the crystal, and determination of the optimum addition amount requires remarkably complicated procedures, thereby raising manufacturing costs.
In cases where reactive gases are used, a phenomenon often occurs in which the reactive gas is dissolved in a melted fluid and is taken as air bubbles into a crystal (Guggenheim, J. appl. Phys. 34, pp. 2482-2485 (1963)). Satisfactory transmission characteristics are not obtained in almost all the cases.